1. Field of the Invention
This invention relates generally to apparatus for treating liquids wherein a liquid-solid is separated in a sedimentation zone and more specifically to an improved means for collecting and removing settled solids from such a zone.
2. Description of the Prior Art
In liquid treatment systems, sedimentation zones are widely employed to separate liquid-solid mixtures into separate liquid and solid phases by gravitational settling of the suspended solids from the liquid-solid mixture. An example is the activated sludge process, wherein activated sludge is mixed with influent wastewater to form a mixed liquor containing the suspended activated sludge solids. After the mixed liquor has been subjected to aeration to effect BOD removal from the wastewater, the mixed liquor is settled in a sedimentation zone to separate out the activated sludge solids, which are returned to the aeration zone, with the separated liquid being discharged from the process system as BOD-reduced effluent.
In the general practice of liquid-solid separation, the sedimentation zone is provided with scraping devices featuring scraper blades which traverse the bottom of the sedimentation basin and move the settled solids to a localized sump or trough means from which the collected solids are withdrawn.
Prior art solids collection and removal devices utilizing scraper blades can be typified by two basic designs -- those in which a scraper blade is supported by a reciprocating bridge which moves along a longitudinal, as for example rectangular or arcuate, tank and those in which the bridge moves continually along an orbital path about the axis of a circular tank or around a central circular dividing wall around a fully extended peripheral sedimentation zone. The devices utilizing scraper blades which have heretofore been employed in circular tanks and in fully extended, i.e., 360.degree., peripheral sedimentation zones are characteristically designed for uni-directional rotation. In these devices, the scraper blades are rigidly connected to the overlying bridge or supporting boom and serve to collect solids and direct same into a solids collection trough adjacent a side wall of the sedimentation zone.
In rectangular or arcuate configuration sedimentation zones, the sedimentation zone is formed by spaced-apart, substantially parallelly aligned side walls and end walls joined to and disposed substantially at right angles with respect to the side walls. In such sedimentation zones, a bridge is typically provided which extends transversely across and above the sedimentation zone. The bridge is driven mechanically in a longitudinal path reciprocatingly and repetitively along the sedimentation zone between the end walls. The scraper blade is attached to the bridge, extending transversely across the sedimentation zone at the bottom thereof with one end adjacent the solids collection trough, extending along one of the side walls, whereby the solids settled in the bottom part of the sedimentation zone are collected and transferred by the blade to the solids collection trough during the travel of the blade along the sedimentation zone in both directions of movement. The scraper blade is typically disposed at an acute angle with respect to the bridge, for maximum solids collection efficiency.
One problem which has arisen in the use of arcuate and rectangular sedimentation zones is the occurrence of unscraped "dead space" adjacent the end walls of the sedimentation zone. Such "dead space" occurs due to the fact that the scraper blade employed in the sedimentation zone is disposed at an acute angle with respect to the reciprocating bridge. Accordingly, a triangular area of unscraped floor surface develops adjacent the end walls of the sedimentation zone. In the unscraped areas, solids accumulate and are not removed from the sedimentation zone. These dead spaces thus represent a loss of collected solids which may be quite critical to system efficiency, as in the activated sludge process where the collected solids are recycled to the aeration zone to effect BOD removal from the influent wastewater.
It may be contemplated that the aforementioned problem of dead space adjacent the end walls of the sedimentation zone may be overcome by the provision of transversely extending solids collection troughs adjacent the end walls of the sedimentation zone. However, this expedient is not suitable in practice inasmuch as the length of the sedimentation zone over which the scraper must operate is very long and in consequence fine sludges tend to be undesirably stirred up and to re-enter suspension during the long traverse of the scraper blade between the end wall regions, thereby lowering solids collection and removal efficiency for the system. In addition, operation in this manner may place heavy mechanical loads on the bridge/scraper assembly which are most desirably avoided in practice to achieve a simple and inexpensive structural configuration.
In practice, the prior art has overcome the occurrence of dead spaces adjacent the end walls of the sedimentation zone by providing "fills" adjacent to the end walls which conform to the triangular shape of the dead space, so that the scraper blade is aligned with the fill surface at the end of its traverse adjacent the end wall. However, the provision of such fills is disadvantageous in that it increases the structural complexity and cost of the sedimentation zone. In addition, since the fills themselves are not scraped, they must be designed to insure that settled solids do not accumulate on their surfaces.
U.S. Pat. No. 3,977,974 discloses means for collecting and removing settled solids from an arcuate sedimentation zone which eliminates the problem of dead space adjacent the end walls of the arcuate sedimentation zone. In the disclosed apparatus, a first cable means joins an inner part of the bridge and an inner part of the scraper blade and second cable means join an outer part of the bridge and an outer part of the scraper blade, for drawing the scraper blade in an arcuate path around the arcuate sedimentation zone in trailing relationship to the points of attachment of the first and second cable means to the bridge. In this arrangement, the cable means nearer the solids collection trough are longer in length than the other cable means such that the scraper blade is disposed at an acute angle with respect to the radially extending bridge during movement of the bridge around the sedimentation zone, whereby the solids settled in the bottom part of the sedimentation zone are collected and transferred by the blade to the solids collection trough during the drawing of the blade around the sedimentation zone in both directions of movement. A third cable means join one of the inner and outer parts of the scraper blade with an opposite part of the bridge, to restrict transverse movement of the scraper blade during movement of the bridge around the sedimentation zone. Means are further provided for shortening the suspended lengths of the first and second cable means at the approach of the bridge to each end wall radial partition, whereby the scraper blade is drawn toward the radial partition for radial alignment therewith, and for increasing the suspended lengths of the first and second cable means after the radial alignment upon reciprocal movement of the bridge toward the other end wall radial partition, so as to eliminate solids accumulation adjacent the end wall radial partitions. Although the apparatus of this patent has proven satisfactory in practice, it requires a number of cable means which are susceptible to breakage. At least three suspension means or cables must be employed, each one representing a potential failure mode for the apparatus.
Accordingly, it is an object of the present invention to provide an improved means for collection and removal of settled solids from the bottom part of a sedimentation zone which eliminates solids accumulation adjacent the end walls of the sedimentation zone.
It is another object of the present invention to provide an improved means of the above type which is mechanically simple and inexpensive to fabricate.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.